Article usage — need help on how to best explain these examples

Question

I'm struggling to find a decent explanation for a student on the following:

'The test is really hard.' (we use definite article to specify which test, assuming listener knows which one we mean). But...

'This is A really hard test.' (we can't use the definite article in this statement and it would look very wrong, but I can't explain why).

Secondly, article rules say that mountains and rivers, lakes etc take the definite article (Eg we went camping in the Dordogne), but we would say "Mount Hood is A volcano". Again, although as a native speaker I understand it myself, I'm unable to explain to the student why this is, in a way that he will understand. He is pre-intermediate and I am used to teaching B2/C1 level learners who rarely ask me such questions!

Any help much appreciated :)

Answer 1

As far as I know, the article the comes from the old English word for that (if something has a the in front of it, you can point at it). So, you can kind of mentally substitute the with that (or those for things that are plural) and if the whole thing still makes sense, then the usage is most likely correct. The indefinite article a/an, again, comes from the old English word for one. I usually think of it like this:

Volvo is a car. It is a car because it is just one of many possible cars that exist.

or

Volvo is a car. It is a car because it is one example of the nebulous idea of car.

Another example:

I'm gonna buy a car. I'm gonna buy just one out of many cars that exist.

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With the, I think, we will typically be able to add some very specific information about the object we are talking about. For example:

I saw the man again. (The man that we met yesterday and his name was John.)

Where are the kids, honey? (The kids that are named Jimmy and Bobby because they are our sons.)

Compare that with examples using a:

I saw a man again. (maybe in my dream) (just a man, one of the class man)

What do kids like most of all, honey? (just kids in general, not our children or our future children)

Answer 2

Your question is one of specific versus general reference and just depends on what you want to say

The test is really hard
this specific test is really hard

This is a really hard test.
Of tests in general, this is really hard.

This is the really hard test.
Of the set of required tests that you need to take, this is the really hard one.

Answer 3

Perhaps it's best to take a walk around and explore the proper context/implication/natural setting for each of the variations of using a/the for both sentences:

The test is really hard.

Implicitly, we're talking about a particular test. We've answered the question "which test is really hard", in a sense. The driver's license test is really hard; the CFA Level III is hard; the MCAT is hard; et cetera.

I think more common in typical usage would be "this/that test is really hard"; it's a (tiny) bit more of a struggle to imagine an actual conversation where this is what's used.

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A test is really hard.

In this exact form, we'll have to do some dancing to make it work (if pressed, I'd say "I have the option to give my students an exam or a speech for their final project; a test is really hard..."), but we can. It's a less common/more obscure version of saying

I find it really hard to take a test.

That is, I struggle with test-taking in general, so I find taking any test to be an ordeal.

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This is a really hard test.

In the world of tests, this one is particularly difficult. There are perhaps many such difficult tests; this test is just one of them.

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This is the really hard test.

I'm a professor and I've created three versions of my final exam. This is the really hard one. The other two are easier on my students. There is no ambiguity, as this test stands alone in its difficulty.

Very similar to:

This is the hardest test.

Answer 4

To answer the second part of your question:

If you use the name of a river, mountain or region, then assuming it is the only one with that name, you are talking about a specific river or mountain, so the is the right article to use.

We went rowing on the Thames
You can go skiing in the Alps
Dubai is on the Arabian peninsula

The Alps is a set of mountains. If you talk about a specific member of the set, you use the. If you talk about a general member of the set, you use a.

We climbed the Matterhorn on our holiday
Mont Blanc is the tallest Alp
I would like to climb an Alp: any Alp would do.

In the sentence

Mount Hood is a volcano

This sentence states that Mount Hood is a member a set called volcanoes. Mount Hood (the subject) is specific, but the subject does not require an article. Within the set of volcanoes (the object) there is nothing special about it- it's a typical volcano, so we use the general article a.

If we talk about a volcano that is specific within the set, we have to use the specific article the:

Mauna Kea is the tallest volcano.
Mount Agung is the nearest volcano to Tulamben.

Answer 5

Let's keep it short and simple:

A guideline that often works is if the speaker assumes his addressee can identify the referent, he will use the definite article. Thus, the speaker could say This is the really hard test when he expects his addressee to be able to identify which (really hard) test he is talking about.

This is the same factor that you used to justify the test in your first sentence.

It also covers the use of a volcano. Here, a volcano is any old garden variety volcano*, not a definite volcano.